The present invention relates to a demodulation circuit, and more particularly relates to an apparatus using a single input signal and integrating all demodulation devices therein.
Wireless communication uses electromagnetic waves as the medium to send a signal (data). This use of electromagnetic waves to carry a signal (data) is called a modulation process. Conversely, retrieving data from an electromagnetic wave is called demodulation. An electromagnetic wave that carries a signal is called a carrier wave and the signal thus carried is called a signal wave or a modulating wave. A modulated carrier wave is called a xe2x80x9cmodulated wavexe2x80x9d. There are three methods of modulation, the xe2x80x9camplitude modulation methodxe2x80x9d, the xe2x80x9cfrequency modulation methodxe2x80x9d, and the xe2x80x9cphase modulation methodxe2x80x9d.
This means that the frequency of the carrier wave changes according to the frequency of the modulating wave in the frequency modulation method. For example, assuming that the signal of the carrier wave is Vc and the signal of the modulating wave is Vs, the frequency modulation wave VFM is as follows:
vc=xcex5c sin xcfx89t(xcfx89=2xcfx80f)
vs=xcex5s cos pt(p=2xcfx80f)
vFM(t)=xcex5c sin(xcfx89t=xcex2F sin pt)
where xcex2F is the index of the frequency modulation.
The frequency modulation method shifts the original frequency value of carrier wave by a value proportional to the frequency of the modulating wave. The process of drawing the modulating wave Vs out from the frequency modulation wave VFM is called frequency demodulation. Reference is made to FIG. 1, a schematic drawing of the demodulator 101 being used to demodulate the modulating wave from the frequency modulation wave.
Different demodulating methods can be used in the demodulator 101. For example, the frequency variation can be transferred to amplified variation, after which an amplified demodulation process is performed to draw out the modulating wave. Alternately, the frequency variation can be transferred to phase variation, after which a phase demodulation process is performed to draw out the modulating wave.
The conventional quadrature demodulator uses the second method to perform the demodulation process. Reference is now made to FIG. 2, a description of the demodulation process. Assuming that the frequency modulation wave is VFM1,
vFM1(t)=xcex51 sin(xcfx89t+xcex2F sin pt)=xcex51 sin xcex1
xcex1=wt+xcex2F sin pt
The quadrature phase-shift apparatus 201 receives the frequency modulation wave VFM1 and generates a quadrature output signal VFM2:
vFM2(t)=xcex52 sin(xcex1+xcfx80/2)
The two waves VFM1 and VFM2 are then multiplied together by the multiplier stage 202 to generate an output signal:                     v        FM1            ⁡              (        t        )              xc3x97                  v        FM2            ⁡              (        t        )              =                    ϵ        1            ⁢      sin      ⁢              xe2x80x83            ⁢      α      xc3x97              ϵ        2            ⁢              sin        ⁡                  (                      α            +                          π              2                                )                      =                            ϵ          1                ⁢        sin        ⁢                  xe2x80x83                ⁢        α        xc3x97                  ϵ          2                ⁢        cos        ⁢                  xe2x80x83                ⁢        α            =                        1          2                ⁢                  ϵ          1                ⁢                  ϵ          2                ⁢        sin        ⁢                  xe2x80x83                ⁢        2        ⁢        α            
From the foregoing description, the phase of the output signal demodulated by the conventional quadrature demodulator gives rise to a second order frequency component (2xcex1). Therefore, it is necessary to uses the low pass filter 203 to eliminate the second order frequency component.
However, the foregoing method has a major drawback. An output wave having the second order frequency component (2xcex1) is generated in the phase when the two waves VFM1 and VFM2 are input into the multiplier stage 202. Therefore, a high quality low pass filter 203 is needed in the conventional demodulator circuit to eliminate the second order frequency component (2xcex1). The low pass filter 203 must be highly selective because the second order frequency component (2xcex1) is very close to the required frequency xcex1 of the low pass filter 203 is required. The high selectivity means that the slope of the frequency response curve is high. Therefore, the low pass filter 203 and the demodulator circuit may not form an integrated circuit because the low pass filter 203 is composed of discrete devices.
FIG. 3 illustrates another demodulation method. The Philips company discloses the method in the U.S. Pat. No. 5,341,107. The in-phase (I) and quadrature (Q) modulated signals are input mutually into the complex circuit 301 to generate two modulated signals having time delay, respectively, in the phase (Id) and quadrature (Qd) signals. Then, the in phase and delay time modulated signal (Id) and the quadrature modulated signal (Q) are multiplied together by the multiplier stage 302 to generate an output signal. At the same time, the quadrature and delay time modulated signal (Qd) and the in phase modulated signal (I) are multiplied together by the multiplier stage 303 to generate an output signal. Then, the two output signals are subtracted from each other in the differential stage 304 to generate the modulating signal Vs.
Although the method provided by the Philips Company may integrate all demodulation devices including the low pass filter into one circuit, two modulated signals, in phase and quadrature ones, are necessary in the input terminals. Therefore, two circuits are needed to process the in phase and quadrature modulated signals at the same time before these two signals are received by the complex circuit 301. This will not only requires additional circuits but also consumes more power.
To resolve the forgoing problems, the main purpose of the present invention is to provide a new design demodulator that eliminates the second order frequency of demodulated signal. Therefore, use of a low pass filter having high selectivity to filter the frequency is no longer necessary. In other words, all demodulation devices and the low pass filter may be combined into an integrated circuit according to the present invention. Moreover, the demodulator of the present invention only requires one modulated input signal to perform the demodulating process. Therefore, only one circuit is required to handle the modulated signal before the demodulator begins to process the signal. This is not similar to Philips"" invention that requires two circuits and so may save power.
The demodulator of the present invention comprises two, the first and the second, I-Q splitters with constant group delay apparatuses. When operated, the first I-Q splitter with constant group delay apparatus first receives the modulated input signal I and then generates two modulated output signals. One is modulated signal Ixcfx841 which is in phase and has delay xcfx841 time compared with I and the other is modulated signal Qxcfx841 which is in quadrature and has delay xcfx841 time compared with I. Then, the second I-Q splitter with constant group delay apparatus receives the modulated signal Ixcfx841 and generates two modulated output signals. One is modulated signal Ixcfx841+xcfx842 which is in phase and has a delay xcfx842 time compared with Ixe2x88x921 and the other is modulated signal Qxcfx841+xcfx842 which is in quadrature and has a delay xcfx842 time compared with Ixcfx841. Next, in phase and having delay xcfx841 time modulated signal Ixcfx841 and in quadrature and having delay xcfx841+xcfx842 time Qxcfx841+xcfx842 are multiplied together by the first multiplier stage to generate an output signal. At the same time, in phase and having delay xcfx841+xcfx842 time modulated signal Ixcfx841+xcfx842 and in quadrature and having delay xcfx841 time modulated signal Qxcfx841 are also multiplied together by the second multiplier stage to generate another output signal. Finally, the two output signals are subtracted from each other in the differential stage to demodulate the modulated signal I. It is noted that xcfx841 is the delay time of the first I-Q splitter with the constant group delay apparatus and xcfx842 is the delay time of the second I-Q splitter with the constant group delay apparatus.
Because the frequency of the signal demodulated by the demodulator of the present invention does not include the second order frequency, it is not necessary to use a low pass filter having high selectivity to filter the frequency. In other words, all demodulation devices and the low pass filter may be combined into an integrated circuit according to the present invention.
On the other hand, the demodulator of the present invention only requires one modulated input signal I to perform the demodulating process. Therefore, only one circuit is required to handle the modulated signal before the demodulator beginning to process the signal. This is different from Philips"" invention requiring two circuits and so may save power.